Investments (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost		$ 7,173,235	$ 7,152,127
Amortized Cost or Cost, Equity securities		143,203	134,013
Unrealized Investment Gains		392,286	374,991
Unrealized Investment Losses		54,771	70,410
Fair Value and Fair Value, Equity		7,510,750	7,456,708
Investment Fair Value, Equity securities		156,975	141,649
OTTI in AOCI	[1]	1,538	1,618
Mortgage-backed securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	[2]	581,346	587,355
Unrealized Investment Gains	[2]	33,904	34,256
Unrealized Investment Losses	[2]	5,443	6,720
Fair Value and Fair Value, Equity	[2]	609,807	614,891
OTTI in AOCI	[1],[2]	0	0
Other, including U.S. Treasury Securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost	[2]	520,718	458,745
Unrealized Investment Gains	[2]	19,809	18,518
Unrealized Investment Losses	[2]	9,219	10,120
Fair Value and Fair Value, Equity	[2]	531,308	467,143
OTTI in AOCI	[1],[2]	0	0
Municipal bonds [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost		1,647,314	1,648,252
Unrealized Investment Gains		148,319	143,733
Unrealized Investment Losses		18,206	22,588
Fair Value and Fair Value, Equity		1,777,427	1,769,397
OTTI in AOCI	[1]	0	0
Foreign government bonds [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost		93,845	93,864
Unrealized Investment Gains		5,897	5,102
Unrealized Investment Losses		56	297
Fair Value and Fair Value, Equity		99,686	98,669
OTTI in AOCI	[1]	0	0
Corporate bonds [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost		2,683,746	2,672,818
Unrealized Investment Gains		162,602	152,229
Unrealized Investment Losses		9,175	14,826
Fair Value and Fair Value, Equity		2,837,173	2,810,221
OTTI in AOCI	[1]	0	0
Other mortgage-backed securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost		1,646,266	1,691,093
Unrealized Investment Gains		21,755	21,153
Unrealized Investment Losses		12,672	15,859
Fair Value and Fair Value, Equity		1,655,349	1,696,387
OTTI in AOCI	[1]	1,538	1,618
Equity securities [Member]
Unrealized gains and losses on fixed maturities and equity securities
Amortized Cost or Cost, Equity securities	[3]	143,203	134,013
Unrealized Investment Gains	[3]	15,743	13,210
Unrealized Investment Losses	[3]	1,971	5,574
Investment Fair Value, Equity securities	[3]	156,975	141,649
OTTI in AOCI	[1],[3]	$ 0	$ 0

[1] Related to securities for which an unrealized loss was bifurcated to distinguish the credit-related portion and the portion driven by other market factors. Represents the amount of OTTI losses in AOCI which was not included in earnings; amounts also include net unrealized investment gains and losses on such impaired securities relating to changes in the fair value of those securities subsequent to the impairment measurement date.

[2] Fair value includes securities issued by Federal National Mortgage Association (“FNMA”) of $283,039 and $272,668; Federal Home Loan Mortgage Corporation (“FHLMC”) of $373,017 and $378,683; and Government National Mortgage Association (“GNMA”) of $112,065 and $115,627 as of March 31, 2017 and December 31, 2016, respectively.

[3] Includes nonredeemable (perpetual) preferred stocks, common stocks and closed-end funds.